On Wed 12-10-16 11:43:37, Michal Hocko wrote:
> On Wed 12-10-16 14:55:24, Anshuman Khandual wrote:
[...]
> > Why we insist on __GFP_THISNODE ?
>
> AFAIU __GFP_THISNODE just overrides the given node to the policy
> nodemask in case the current node is not part of that node mask. In
> other words we are ignoring the given node and use what the policy says.
> I can see how this can be confusing especially when confronting the
> documentation:
>
> * __GFP_THISNODE forces the allocation to be satisified from the requested
> * node with no fallbacks or placement policy enforcements.
You made me think and look into this deeper. I came to the conclusion
that this is actually a relict from the past. policy_zonelist is called
only from 3 places:
- huge_zonelist - never should do __GFP_THISNODE when going this path
- alloc_pages_vma - which shouldn't depend on __GFP_THISNODE either
- alloc_pages_current - which uses default_policy id __GFP_THISNODE is
used
So AFAICS this is essentially a dead code or I am missing something. Mel
do you remember why we needed it in the past? I would be really tempted
to just get rid of this confusing code and this instead:
---
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index ad1c96ac313c..98beec47bba9 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1679,25 +1679,17 @@ static nodemask_t *policy_nodemask(gfp_t gfp, struct mempolicy *policy)
static struct zonelist *policy_zonelist(gfp_t gfp, struct mempolicy *policy,
int nd)
{
- switch (policy->mode) {
- case MPOL_PREFERRED:
- if (!(policy->flags & MPOL_F_LOCAL))
- nd = policy->v.preferred_node;
- break;
- case MPOL_BIND:
+ if (policy->mode == MPOL_PREFERRED && !(policy->flags & MPOL_F_LOCAL))
+ nd = policy->v.preferred_node;
+ else {
/*
- * Normally, MPOL_BIND allocations are node-local within the
- * allowed nodemask. However, if __GFP_THISNODE is set and the
- * current node isn't part of the mask, we use the zonelist for
- * the first node in the mask instead.
+ * __GFP_THISNODE shouldn't even be used with the bind policy because
+ * we might easily break the expectation to stay on the requested node
+ * and not break the policy.
*/
- if (unlikely(gfp & __GFP_THISNODE) &&
- unlikely(!node_isset(nd, policy->v.nodes)))
- nd = first_node(policy->v.nodes);
- break;
- default:
- BUG();
+ WARN_ON_ONCE(polic->mode == MPOL_BIND && (gfp && __GFP_THISNODE));
}
+
return node_zonelist(nd, gfp);
}
--
Michal Hocko
SUSE Labs
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